There are various ways to propel a boat along the water. The two primary ways are to use motors to spin a propeller and the wind to provide force to a sail or other mechanism. In conventional uses, wind propelled vehicles often use sails to deflect the wind. Sails generate forward thrust by accelerating wind toward the rear of the vehicle. There are various forms of wind-propelled vehicles, including “kite” powered vehicles (such as kite-boards), windmill-powered vehicles and parachute-powered vehicles. Kite-powered vehicles have some drawbacks in application to larger boats including very high operator workload and the potential of “crashing” the kite into the water. Windmill-powered boats generally connect the windmill to an underwater propeller, providing a kind of hybrid system that is too far from the subject system to include. In each application of interest, the propulsive means (e.g. sail or wing) operates in conjunction with a second mechanism that constrains the lateral motion of the vehicle. Such constraining mechanisms include keels, centerboards, wheels, and ice blades. The primary purpose of a sail is to provide thrust to offset the hull's water and air drag as well as drag from the keel. The keel is needed to offset side force, a byproduct of the sail or wing in almost all sailing conditions.
FIG. 1 is used to illustrate some basic force vectors that act upon a typical sailboat. In FIG. 1, aerodynamic forces are created by the relative motion of the boat and wind. The wind speed experienced by the boat is a vector sum of the wind speed relative to the fixed surface and the boat speed relative to that surface. This is illustrated in FIG. 1. In FIG. 1, the wind velocity vector relative to the surface is labelled Vwind, the boat velocity vector relative to the surface is labelled Vboat, the sum of these two vectors is called “apparent wind” and is labelled Vapparent, the angle of the wind relative to the surface path of the boat is labelled Awind, and the angle of the apparent wind relative to the surface path of the boat is labelled Aapparent. Most sailboats have a slight offset in the angle between the hull axis and the boat path due to the needed angle of attack of the keel. This angle is labelled Aslip.
In typical use, the angle of the apparent wind falls between the boat path and the wind relative to the surface. Further, higher boat speed reduces the apparent wind angle. Conversely, greater wind speed increases the apparent wind angle. The apparent wind speed is equal to wind speed when the sum of the apparent wind angle and the wind angle is 180°. To achieve this relationship as the boat slows down, approaching zero speed, the wind angle approaches 90° from behind (a slight tailwind component). To achieve this relationship as the boat speeds up, the wind angle increases beyond 90° (an increasing tailwind component). When the sum of the apparent wind angle and the wind angle is less than 180°, the apparent wind speed is greater than the wind speed. When the sum of the apparent wind angle and the wind angle is greater than 180°, the apparent wind speed is less than the wind speed. Greater wind speed results in greater apparent wind speed and vice-versa.
FIG. 2A is provided to illustrate lift, drag, and net force on a sail caused by the apparent wind. The apparent wind acts on the sail or wing to create lift and drag. In FIG. 2A, as illustrated, a lift force can be generated by the sail. This force is perpendicular to the apparent wind angle, by definition. That is, by convention, lift is perpendicular to the freestream flow which, for sailboats, is equivalent to the apparent wind. The wing also generates a drag force. This force is by definition parallel to the apparent wind angle. Lift and drag vectors, being orthogonal, may be summed using the Pythagorean equation to find the magnitude of Net Force. Lift and drag force are proportional to the apparent wind speed squared and wing area. Wing lift may be modulated by adjusting lift coefficient. This may be controlled by adjusting wing angle of attack. Drag force adds to the Net Force but increases the angle of the net force (swings the net force vector aft) when the apparent wind angle is less than 90 degrees. The wing angle of attack shown in FIG. 2A may be reversed so that the lift vector points in the opposite direction. This is illustrated in FIG. 2B. In this case, both the lift vector and drag vector point aft, resulting in a strong braking action.
FIG. 3 is used to illustrate thrust and sideforce. The Net Force vector may be resolved into two vectors. These are Thrust, a force parallel to the path of the boat on the water surface, and Sideforce, a force perpendicular to the boat's path. Thrust drives the boat forward. In a steady-state condition, the wing's thrust equals the sum of the drag from the hull moving forward through the water and the air as well as the drag of the keel (or centerboard) dragging through the water. Increased thrust generally results in increased boat speed and vice-versa. Sideforce is a byproduct of the wing thrust process that is produced in almost all sailing conditions. Exceptions include when the boat is headed directly downwind.
As the net force angle increases (sweeps aft), the thrust magnitude diminishes and may reverse. The net force angle is increased by increased drag and by smaller apparent wind angles. At smaller apparent wind angles, wing drag strongly subtracts from thrust. At higher apparent wind angles, drag has a weaker effect on thrust. As the apparent wind angle moves aft, thrust increases and sideforce tends to diminish.
FIG. 4 illustrates keel forces. Most wind-powered vehicles have a mechanism that opposes wing sideforce with an equal and opposite force so that the vehicle may proceed along its intended path. In the case of sailboats, most have a keel or centerboard projecting below the hull. Landsailers use wheels and ice boats use blades. Keels and centerboards act as a wing in the water to generate lift to oppose the sideforce. They also generate drag. A force diagram is provided in FIG. 4.
In FIG. 4, keel lift opposes sideforce with and equal but opposite force. It also creates some drag to be overcome by the wing thrust. Keel lift is, by definition, perpendicular to the boat's path through the water. Keel drag is, by definition, parallel to the boat's path. To generate lift, the keel typically must operate at an angle of attack to the path (slip angle). Since the keel is typically fixed to the hull (and the boat is laterally symmetric), the hull must also pass through the water at the same slip angle as the keel. Note that this diagram assumes that all sideforce is provided by the keel. This may not be true—the hull may also provide some sideforce because it is slipping through the water at a slight angle.
Conventional sailboats can have some limitations. Upwind speed (“velocity made good”) is inhibited by a sail's relatively high drag. When pointing upwind, sail drag is in near opposition to thrust, so thrust is diminished by drag. High drag is a product of the imperfect airfoils formed by sails, drag from the mast and rigging, and an inefficient spanwise distribution of lift. Velocity made good (VMG) is the boat's speed component in the opposite direction of the wind (directly into the wind), a key measure of sailboat performance. For a given set of conditions there is an optimum heading that maximizes VMG by providing the best combination of upwind angle and boat speed. High sail drag reduces thrust and boat speed at high upwind angles. This results in the optimum heading providing both reduced boat speed and upwind angle, hurting VMG. A favorable VMG is important because sailboats spend a considerable fraction of many round-trip journeys sailing at or close to the optimum VMG condition. Further, large heading changes require careful coordination of sail adjustments and the boat heading. Emergency maneuvers, for instance to recover a man overboard, can be difficult and time-consuming to execute safely.
Gusty wind conditions place an additional workload on the crew to rapidly adjust sail trim. Gusts in wind strength or direction can endanger the boat with excessive heeling moment, potentially resulting in capsizing the boat. Larger sailboats have many separate controls and usually require a multiple-person crew to operate. Significant effort is required on each voyage to prepare the boat to sail. Preparations include selecting sails, raising or unfurling sails and trimming. This often must be done while underway because the boat's orientation in the slip with respect to the wind may be unfavorable. Conversely, significant work is required to lower and stow sails before berthing or tying up.
Substantial sail configuration changes are often required to accommodate wide changes in wind conditions. This can take a lot of time and work. Adjustments to sail configuration and to sail trim involves high loads and significant power. Connecting and disconnecting sheets (lines to sails) may be required to change sails or even just to tack. Alternatively, sheets may need to be disconnected from one set of winches and reconnected to other winches, a laborious and demanding task. In light wind conditions when motoring, sails are usually lowered, especially when motoring upwind. Aerodynamic drag from the mast and rigging is significant, especially when motoring upwind, resulting in additional fuel consumption. Also, drag from the mast and rigging may contribute to dragging an anchor or mooring in high wind conditions.
Operation of the boat requires access to the deck to operate the sails. Most sailboats are operated from an open or partially open cockpit with ready access to most of the sail controls and to the deck for access to the sails themselves and to additional sail controls. Sailboats with interchangeable sails may require a sail storage area with access to the deck. Sail rigs are typically anchored to the hull in multiple places and impose large loads on the hull. Rig attach points may include the forestay, main stays, back stay, the boom yang, the mainsheet (or traveler), the jib sheet, and winches that control the sails. These many load points increase the weight and cost of the hull and conflict with other potential uses of the deck (such as walking and relaxing). Sometimes, cantilevered masts are used. The dimensions of such masts are a compromise between aerodynamic efficiency and structural weight. That is, they are both heavier and less efficient than masts otherwise separately optimized for aerodynamics and structure. Complexity of the sail and rigging system results in many potential points of failure. A single failure may disable the sail.
It is with respect to these and other considerations that the disclosure made herein is presented.